Lecture 12 part 2 Finish Snowball Earth

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Lecture 12 (part 2)Finish Snowball Earth
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Hard Snowball EarthIce Thickness
Ts ? -27o C (Hyde et al., 2000)
?z
Fg
Toc ? 0oC
Let k thermal conductivity of ice ?z
ice thickness ?T Toc Ts Fg geo
thermal heat flux
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Hard Snowball (cont.)
The diffusive heat flux is Fg k?T / ?z
Solving for ?z gives ?z k?T /
Fg ? (2.5 W/m/K)?(27 K)/(60?10-3 W/m2)
?z ? 1100 m ? No sunlight gets through
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How did photosynthetic life survive the Snowball
Earth?
Refugia such as Iceland? Tidal cracks, meltwater
ponds, tropical polynyas? (Hoffman and Schrag,
Terra Nova, 2002) Thin ice model (C. McKay, GRL
, 2000) Tropical ice remains thin due to penetra
tion of sunlight
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McMurdo Sound dive hole
Ice thickness 2.5-3 m
Courtesy of Dale Andersen
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Lake Bonney (Taylor Valley)
Photosynthetic life thrives beneath 5 m of ice
Courtesy of Dale Andersen
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Thin ice model
If the ice were thin (1-2 m), then it would have
let an appreciable amount of sunlight through
That energy would have had to escape by
conduction through the ice The ice could have bee
n maintained in a thin state by this heat flux
because the solar flux is much higher than the
geothermal heat flux Geothermal heat flux Fg 6
0 mW/m2 0.06 W/m2 Solar heat flux
FS 1370 W/m2 ? (1/4) ? (1-0.3)
? 240 W/m2
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Ice Transmissivity (400-700 nm)
Possible solution at equator
Photosynthetic limit
C. McKay, GRL (2000)
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Duration of the Snowball Events

• If a Snowball was truly global then it would take
  10-20 million years to accumulate huge amount of
  CO2 necessary to defreeze Earth
  
• Interplanetary dust would keep falling on the
  already glaciated planet
  
• After deglaciation all the interplanetary dust
  would be flushed into the ocean and deposited in
  the sediments

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Iridium and interplanetary material

• Ir is present in all meteorites
• Siderophile (iron-loving) element dissolves in
  molten iron
  
• Ir is depleted in the Earths mantle and Earths
  crust because it migrated into the core with iron
  (Fe).
  
• Meteorites and dust particles are enriched in Ir
  comparing to the Earths crust

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On Earth Iridium is in the core.
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Bodiselitsch et al. (2005)
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Summary Evidence for Snowball events

• Global distribution of glacial deposits
• Deposition of glacial layers close to
  paleo-equator
  
• Deposition of pos t-glacial cap carbonates
• Increased concentrations of Ir at the base of
  Sturtian and Marinoan cap carbonates